The isomorphism problem for classes of computable fields
Wesley Calvert
备注：
15 pages, updated references, general revision, previously entitled "The isomorphism problem for familiar classes of computable structures I", long version of published paper
Theories of classification distinguish classes with some good structure theorem from those for which none is possible. Some classes (dense linear orders, for instance) are non-classifiable in general, but are classifiable when we consider only countable members. This paper explores such a notion for classes of computable structures by working out several examples. One motivation is to see whether some classes whose set of countable models is very complex become classifiable when we consider only computable members.
We follow recent work by Goncharov and Knight in using the degree of the isomorphism problem for a class to distinguish classifiable classes from non- classifiable. For some classes (undirected graphs, fields of fixed characteristic, and real closed fields) we show that the isomorphism problem is Σ^1_1 complete (the maximum possible), and for others it is of relatively low complexity. For instance, for algebraically closed fields, archimedean real closed fields, and vector spaces, we show that the isomorphism problem is Π^0_3 complete.
Computable Isomorphisms for Certain Classes of Infinite Graphs
我们研究了（2，1）：1结构，它由一个可数集$A$和一个函数$f: A \to A$组成，使得对于$A$中的每个元素$x$，$f$将$A$中的一个元素或两个元素映射到$x$ .这些结构扩展了Cenzer、Harizanov和Remmel研究的注入结构、2：1结构和（2，0）：1结构的概念，所有这些结构都可以被认为是无限有向图。我们研究了（2，1）：1结构的各种可计算性理论性质，最值得注意的是可计算范畴性。我们说一个结构$\mathcal{A}$是可计算范畴的，如果在$\mathcal{A}$的任何两个可计算副本之间存在可计算同构。给出了（2，1）：1结构可计算范畴的一个充分条件，并给出了具有不同可计算性理论性质的（2，1）：1结构的一些例子.
Recursively Enumerably Representable Classes and Computable Versions of the Fundamental Theorem of Statistical Learning
Institute for Symbolic Artificial Intelligence, Johannes Kepler University Linz（约翰内斯·开普勒大学林茨分校符号人工智能研究所）；Institute for the Interdisciplinary Study of Language Evolution, University of Zurich（苏黎世大学语言演化跨学科研究所）；Department of Mathematics and Statistics, University of Konstanz（康斯坦茨大学数学与统计学系）
